The present invention relates to a bulk metal chip resistor, and particularly to a bulk metal chip resistor that can be surface mounted on a circuit board.
Bulk metal resistors have been known in the prior art, and an example of such a resistor is shown in U.S. Pat. No. 4,467,311. The resistor in that patent includes a flat metal plate having a plurality of slots extending inwardly from its lateral edges. A pair of electrical leads are welded or otherwise operatively secured to the opposite ends of the plate.
In prior art axial lead resistors such as shown in U.S. Pat. No. 4,467,311, the bulk metal resistance element or plate is generally formed from a material having a low temperature coefficient of resistance (TCR) often in the range of 25 ppm/.degree. C. The axial leads welded to the resistor are usually formed of copper or other highly conductive metals having a very high TCR which is generally above 150 ppm/.degree. C.
The axial leads of prior art resistors influence both the overall resistance value and the overall TCR of the resistor. The leads affect the overall TCR of the resistor in direct proportion to the ratio of the resistance value of the leads to the resistance value of the resistance element. In lower value resistors (for example 1 ohm or less) the resistance value of the long axial leads is high compared to the resistance value of the low value resistance element. As a result, in these lower value resistors, the leads significantly raise the overall TCR of the resistor above the lower TCR of the resistance element.
Another disadvantage of prior art axial lead resistors is in the manner in which heat is dissipated through the leads of the resistor to the board on which the resistor is mounted. The length of the leads retards the conduction of heat thereby causing the resistor to have a lower than desired wattage rating for any given size.